Stern post

[soldthehouse]

What are the pros and cons of a pointed swim as opposed to a flat end plate where the stern tube exits the hull, is cavitation altered considerably or is it just paper theoretics and pub talk gone mad.

[blackrose]

What are the pros and cons of a pointed swim as opposed to a flat end plate where the stern tube exits the hull, is cavitation altered considerably or is it just paper theoretics and pub talk gone mad.

 

I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub!

 

The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides.

I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it.

Personally at the speeds we travel I doubt it makes much difference.

Edited August 17, 2009 by blackrose

[mayalld]

I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub!

 

The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides.

I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it.

Personally at the speeds we travel I doubt it makes much difference.

 

The question related to the design of the STERN, not the bow.

 

Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat.

[blackrose]

The question related to the design of the STERN, not the bow.

 

Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat.

Ah, that'll teach me not to respond to posts on my mobile phone, I thought it said stem post!

 

However my original point about minor differences in hull design making little difference at the speeds we travel still stands. Canal boats are not high performance craft.

Edited August 17, 2009 by blackrose

[Albion]

The question related to the design of the STERN, not the bow.

 

Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat.

 

It is clearly beneficial to deliver the water to the prop as smoothly as possible and as much as possible. A blunt-ended stern swim isn't going to do this as well as a pointed one as there will be some turbulence when the water leaves the tip. However, how much difference this makes on a narrow boat is difficult to quantify without scientific tests I reckon. Suffice it to say that the more pointed the swim the better IMO.

Roger

[Timleech]

It is clearly beneficial to deliver the water to the prop as smoothly as possible and as much as possible. A blunt-ended stern swim isn't going to do this as well as a pointed one as there will be some turbulence when the water leaves the tip. However, how much difference this makes on a narrow boat is difficult to quantify without scientific tests I reckon. Suffice it to say that the more pointed the swim the better IMO.

Roger

 

Agreed. With the blunt stern post, the distance between prop & stern post will become very significant.

 

Tim

[RLWP]

Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through?

 

(awaits input from a boat builder)

 

Richard

[furnessvale]

Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through?

 

(awaits input from a boat builder)

 

Richard

 

Perhaps boat builders could finish with a half round tube. The length of weld would be exactly the same and the finished job would look like a Woolwich boat at least.

 

George ex nb Alton retired

[hughc]

Of course narrow boats are high performance craft. They are not fast in restricted channels though many well designed hulls will reach their maximum displacement speed in open water. Because narrow boats fit the channel so closely every small improvement in how they swim will make a noticeable difference. Many modern cheap clones swim like bricks because the design input was minimal. Bluff bowed and short swims on GUCC boats didn't affect their speed because the overall underwater shape was considered as a whole by qualified naval architectts and the subtle curvature aided water flow in narrow channnels. Flat ended sternposts create considerable turbulence which affects propeller efficiency to a large degree. Wake reduction plays an important part in the choice of propeller. If the prop. is in free water then the speed of the water into it is the speed of the boat. The more restricted and turbulent the flow the greater the difference between boat speed and propeller speed because water is being dragged along by friction. Warren gives a range of wake reduction from 0% in free water to 30% for a heavily masked prop. Performance is not related solely to speed and a boat that swims well leaving little wash is a joy to steer. Regards, HughC.

[soldthehouse]

I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub!

 

The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides.

I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it.

Personally at the speeds we travel I doubt it makes much difference.

It is the stern post i am enquiring about not the bow

 

Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through?

 

(awaits input from a boat builder)

 

Richard

I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input

 

Of course narrow boats are high performance craft. They are not fast in restricted channels though many well designed hulls will reach their maximum displacement speed in open water. Because narrow boats fit the channel so closely every small improvement in how they swim will make a noticeable difference. Many modern cheap clones swim like bricks because the design input was minimal. Bluff bowed and short swims on GUCC boats didn't affect their speed because the overall underwater shape was considered as a whole by qualified naval architectts and the subtle curvature aided water flow in narrow channnels. Flat ended sternposts create considerable turbulence which affects propeller efficiency to a large degree. Wake reduction plays an important part in the choice of propeller. If the prop. is in free water then the speed of the water into it is the speed of the boat. The more restricted and turbulent the flow the greater the difference between boat speed and propeller speed because water is being dragged along by friction. Warren gives a range of wake reduction from 0% in free water to 30% for a heavily masked prop. Performance is not related solely to speed and a boat that swims well leaving little wash is a joy to steer. Regards, HughC.

It will have to be pointy then

[BEngo]

It is the stern post i am enquiring about not the bow

 

 

I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input

 

 

It will have to be pointy then

 

 

If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits.

 

Regards

N

 

 

It is the stern post i am enquiring about not the bow

 

 

I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input

 

 

It will have to be pointy then

 

 

If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits.

 

Regards

N

[nb Innisfree]

With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged

 

I haven't done this myself but seen it done

[Timleech]

With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged

 

I haven't done this myself but seen it done

 

Unless it's a very blunt swim, I would expect you to run out of depth with the (ordinary) hole saw long before you get to full diameter/width.

 

Tim

[RLWP]

Unless it's a very blunt swim, I would expect you to run out of depth with the (ordinary) hole saw long before you get to full diameter/width.

 

Tim

 

 

Arbour extensions

 

Richard

[Iain_S]

 

Arbour extensions

 

Richard

 

But once you've reached the depth of the actual hole saw, you can't go further.

(Unless you can remove the core from the initial cut, which in this case would also remove any means of locating the saw)

 

Iain

[RLWP]

But once you've reached the depth of the actual hole saw, you can't go further.

(Unless you can remove the core from the initial cut, which in this case would also remove any means of locating the saw)

 

Iain

 

Damn. He's right you know...

 

Richard

[nb Innisfree]

Damn. He's right you know...

 

Richard

 

Yes it was quite a blunt swim, saw him start it and later on saw the result which was very neat, didn't watch him throughout, but I recall him saying it was no problem.

 

Must admit I can't remember if it was a longer saw or not

Edited August 18, 2009 by nb Innisfree

[Timleech]

Yes it was quite a blunt swim, saw him start it and later on saw the result which was very neat, didn't watch him throughout, but I recall him saying it was no problem.

 

Must admit I can't remember if it was a longer saw or not

 

I've never seen 'deep' holesaws. You can get annular (Rotabroach) cutters which go a bit deeper than your normal holesaw but from memory even those only go to maybe 60mm deep - plus the sort of size needed for sterngear would cost more than the sterngear!!

 

Tim

[Swallowman]

Damn. He's right you know...

 

Richard

No he's not, you just use the outside of the arbour to locate it into the bit you have already cut out. I have drilled through 2" timber using a 1/2" deep hole saw. you just take the cores out as you go. You just have to make sure you have a decent hole saw to start with; not one of those dangerous things with different diameter curved blades that fit into a single boss.

[Iain_S]

No he's not, you just use the outside of the arbour to locate it into the bit you have already cut out. I have drilled through 2" timber using a 1/2" deep hole saw. you just take the cores out as you go. You just have to make sure you have a decent hole saw to start with; not one of those dangerous things with different diameter curved blades that fit into a single boss.

 

I don't think that would work from the outside, as there wouldn't be a full circle to locate the saw.Might be possible from the inside, using the pilot drill for location.

 

Iain

[soldthehouse]

If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits.

 

Regards

N

 

 

 

 

 

If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits.

 

Regards

N

Are you saying in effect a flat stern post and then fixing a fairing to smooth the flow of water rearwards towards the prop, if so BRILLIANT if not Ive just thought of something briliant

[nb Innisfree]

With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged

 

I haven't done this myself but seen it done

 

Apparently the hole can be finished from the inside

[soldthehouse]

Apparently the hole can be finished from the inside

First thoughts are is it done in stages and if so how, my powers of comprehension seem to be diminishing after years of solvent abuse at work or perhaps its the onset of the big `A`

[nb Innisfree]

First thoughts are is it done in stages and if so how, my powers of comprehension seem to be diminishing after years of solvent abuse at work or perhaps its the onset of the big `A`

 

First, drill pilot hole from outside and then use hole saw to its max depth, go into engine room and use saw in the same hole to finish cutting. Done.

 

You need to go very slowly and keep drill steady, maybe need a longer drill bit for second stage and perhaps a right angled drive if space is tight.

[soldthehouse]

First, drill pilot hole from outside and then use hole saw to its max depth, go into engine room and use saw in the same hole to finish cutting. Done.

 

You need to go very slowly and keep drill steady, maybe need a longer drill bit for second stage and perhaps a right angled drive if space is tight.

Thanks, there is so much going on at the moment I seem to have a wall up against incoming info, perhaps its a safety thing, thanks for that, regards, stuart